In general, silica is excellent in mechanical strength, has high chemical stability, and is easily available with various shapes, average particle diameters, and particle size distributions. In addition, when a surface of silica is subjected to hydrophobizing treatment, its affinity for a resin can be enhanced to improve dispersibility in the resin and fluidity of the resin. Accordingly, silica is used in various applications, such as an external additive for toner, and fillers for various rubbers, resins, semiconductor encapsulation materials, and films.
Hydrophobized spherical silica, which has a surface subjected to the hydrophobizing treatment, is generally obtained by subjecting a surface of spherical silica obtained by a flame aerosol processe or sol-gel method to the hydrophobizing treatment. The hydrophobized spherical silica obtained by the sol-gel method is known to be as follows as compared to the hydrophobized spherical silica obtained by the flame aerosol processe. That is, the former silica has a narrow width of a particle size distribution, i.e., uniformized particle diameters, and hence is easily used for design of a filling material. However, the silica has a silanol group inside a particle, and hence, even when its surface is subjected to the hydrophobizing treatment, the silica is liable to absorb moisture under high temperature and high humidity, resulting in poor environmental stability.
As one of fine particles to be obtained by the sol-gel method, there is a report of a spherical polyalkylsilsesquioxane fine particle manufactured by hydrolyzing an alkyltrialkoxysilane serving as a raw material. The polyalkylsilsesquioxane fine particle has hydrophobic alkyl groups on its particle surface and inside itself. Accordingly, the polyalkylsilsesquioxane fine particle hardly absorbs moisture, resulting in excellent environmental stability, and hence is very useful. As a method of manufacturing such polymethylsilsesquioxane particle, for example, in Patent Literature 1, there is a report that, for use in an external additive for toner, a truly spherical polymethylsilsesquioxane particle is obtained by hydrolyzing and condensing a methyltrialkoxysilane under a basic atmosphere. In addition, in Patent Literature 2, there is a report of a water or water/organic solvent dispersion liquid containing a spherical polyorganosilsesquioxane fine particle having a particle diameter of about 0.1 μm, and a fine particle collected by centrifuging the dispersion liquid, for use in an additive for a paint or a cosmetic.
In manufacture of the polymethylsilsesquioxane particle by the sol-gel method, stability of a reaction is not sufficient, and hence it is difficult to obtain a particle having a small particle diameter. For example, in Patent Literature 1, there is a disclosure that a truly spherical polymethylsilsesquioxane particle having a particle diameter of from 0.1 μm to 10 μm is obtained, but in Examples, there is only a disclosure that particles having particle diameters of 3.0 μm and 1.9 μm have been obtained.